1. Field of the Invention
The invention relates to the field of thermal interface materials (TIMs) for use in electronic devices or other thermal management applications that require rapid dissipation of heat. Examples of devices needing TIMs include computers, telecommunications, space, military, and medical apparatus.
2. Related Art
Power dissipation in electronic devices is projected to increase significantly over the next ten years to the range of 100-150 Watts per cm2 for high performance applications1. This increase in power represents a major challenge to systems integration since the maximum device temperature needs to be around 100° C. An additional concern is that leakage currents may also significantly increase as the interconnect size continues to decrease into the nanometer realm. Leakage currents will increase the power dissipation levels well beyond the 150 W/cm2 range. Thermal management is a major hurdle in the development of faster processors. In a typical chip heat sink assembly, the highest resistance to heat flow comes from the thermal interface material. Typically, the thermal conductivity of a thermal interface material ranges from 1-4 W/mK. One of the ways to increase thermal performance is to improve the thermal conductivity of the thermal interface material. Many concepts have emerged for increasing the thermal conductivity of thermal adhesives and pastes. A widely used approach is to add micron size, highly conductive filler particles in the matrix of the thermal interface material. Another alternative is to use carbon nanotubes. Nanotubes have unique properties as discussed by Iijima20 and Berber et al.3 have reported nanotubes have measured high electrical and thermal conductivities (around 6600 W/mK at room temperature) for carbon nanotubes. These can be placed in the thermal interface material to provide a low heat resistance path through the thermal interface material, significantly improving the thermal conductivity of the TIM. See, Doctoral Dissertation of Anand Hasmukh Desai “Thermal Management Of Small Scale Electronic Systems”, Binghamton University, State University of New York, 2006, the entirety of which is expressly incorporated herein by reference.
Typical thermal interface materials used in production today include thermal greases and adhesives, thermal gels, phase change materials, and low melt point solders such as Indium2. The thermal conductivity for these materials ranges from about 3 (grease and adhesives) to 30 (solders) W/mK. The minimum required thickness of the thermal interface directly impacts the resistance, and varies considerably between these materials. For example, solder TIM solutions need to be considerably thicker than thermal grease due to thermo-mechanical issues.
Carbon nanotubes (CNTs) are promising new materials exhibiting extraordinary thermal properties, when grown on a device requiring a thermal interface. Theoretical calculations predict an unusually high value of phonon-dominated thermal conductivity at ca. 6600 W/mK3,4, while experimental measurements on individual CNTs confirms a range of 3000-8000 W/(mK) at the room temperature5,6. While the exact values and their validation are still under debate, there is little doubt that the extremely high thermal conductivity of CNTs offers the possibility of using CNTs as TIM in electronics packaging to satisfy the increasing power dissipation challenge.